The present invention relates to an accelerator arranged to circulate a charged particle beam for boosting the beam energy and then to extract the beam and a medical apparatus to which the accelerator is applied. In particular, the invention relates to a small-sized accelerator which is preferable to easily obtaining the excellent charged particle beam with a constant beam diameter and a medical apparatus to which the small-sized accelerator is applied.
The conventional accelerator as shown in FIG. 1 is arranged to accelerate a charged particle beam and then to extract and transport the accelerated beam so that the beam may be used for a physical experiment or a medical purpose. The charged particle beam, which is injected from an accelerator 34 at the previous stage and then is introduced inside of a circular accelerator through the effect of an injector unit 15 and an injecting pulse electromagnet 35, is circulated along a beam central orbit 1 located at the center of a vacuum duct 10 as the beam is kept betatron-oscillated. Such a beam circulating type accelerator is ordinarily referred to as a circular accelerator. When the circular accelerator operates to extract the charged particle beam, the betatron oscillations occurring on the horizontal plane of the beam are resonated through the effect of a focusing quadrupole magnet 5, a defocusing quadrupole magnet 6 and a multipole magnet for exciting resonance 9 for increasing the amplitude of the betatron oscillations, so that the charged particle beam may be extracted from an extracting deflector 4, thereby utilizing the charged particle beam in a medical treatment room or a laboratory 33. Herein and hereafter, the term "magnet" refers to an electromagnet.
The focusing quadruple magnet 5 provides a horizontally focusing effect and a vertically defocusing effect. That is, if the magnet 5 is considered as an optical system, the focusing quadrupole magnet 5 horizontally corresponds to a convex lens or vertically corresponds to a concave lens. Likewise, the defocusing quadrupole magnet 6 provides a horizontally defocusing effect, that is, horizontally corresponds to a concave lens and a vertically focusing effect, that is, vertically corresponds to a convex lens. If the particles are resonated, the amplitude of the betatron oscillations of the particles is increased. To allow the particles to be extracted from the extracting deflector 4 without collision against the vacuum duct 10, there are provided extracting bump magnets 61 and 62 in the conventional accelerator.
The resonance of betatron oscillations is discussed in AIP Conference Proceedings, No. 127, 1983, pp. 52 to 61. This is a phenomenon to be discussed below. The charged particles are circulated as the particles are kept horizontally and vertically oscillated. This is called as betatron oscillations. The number of betatron oscillations per one circulation of a circular orbit is called as a tune. In a case that the tune is adjusted to come closer to an integer +1/3, an integer +2/3 or an integer +1/2 and at once a multipole magnetic field for exciting resonance is excited, among lots of charged particles being circulated, the charged particles having a larger amplitude of the betatron oscillations than a certain level abruptly increase their amplitude. This phenomenon is referred to as resonance of betatron oscillations. Further, the resonance given if the tune is adjusted to come closer to an integer +1/2 is referred to as second order resonance. The resonance given if the tune is adjusted to come closer to an integer +1/3 or an integer +2/3 is referred to as third order resonance. The border of resonance occurrence is referred to as stability limit of resonance. The magnitude of the stability limit changes depending on the strength of multipole magnetic field for exciting resonance and the value of a decimal part of the tune. The value of the tune is controlled by the intensity of the quadrupole magnetic field.
The later description will be expanded based on the case of the third resonance, that is, the case that the tune is adjusted to come closer to an integer +1/3. That is, assuming that the decimal part of the tune is .DELTA..nu. (=0.33), in the multipole magnet for exciting resonance, the displacement of the betatron oscillations is substantially made equal at each of about 1/.DELTA..nu. circulation. If .DELTA..nu. is equal to about 0.33, by applying such a magnetic field as effectively increasing the betatron oscillations to a beam of m-th circulation, (m+1)th circulation, or (m+2)th circulation, the beam displacement of m-th circulation is substantially same as that of (m+3)th circulation. Likewise, the beam displacement of (m+1)th circulation is substantially same as that of (m+4)th circulation and the beam displacement of (m+2)th is substantially same as that of (m+5)th. Hence, the amplitude of the betatron oscillations is remarkably increased. In particular, if .DELTA..nu. comes closer and closer to 1/3, the same displacement of the betatron oscillations takes place at each of three circulations. The multipole magnetic field for exciting resonance is made more effective so that the amplitude of the betatron amplitude is likely to abruptly increase. That is, the stability limit of resonance is made smaller as the deviation of the tune from an integer +1/3 is made smaller and as the multipole magnetic field for exciting resonance is made stronger. As such, the conventional apparatus is arranged to take the steps of adjusting the tune to come closer to an integer +1/3, resonating the charged particles having a larger amplitude of betatron oscillations, selected among the charged particles being circulated, then making the tune come far closer to an integer +1/3 for reducing the stability limit of resonance, and thereby resonating the charged particles having a smaller amplitude of betatron oscillations. The tune control is executed by controlling the strength of the magnetic field of the quadrupole magnets 5 and 6 provided on the circular orbit shown in FIG. 1, that is, the current of the quadrupole magnets 5 and 6.
The particles in which the betatron oscillations are resonated are likely to increase their oscillation amplitudes and reduce a distance between the inner wall of the vacuum duct 10 and the particles as the particles are circulating more and more and more. The extracting bump magnets 61 and 62 are used for shifting the central orbit 1 of the oscillated beam locally toward the extracting deflector 4 before the extraction in order that the beam may be taken out of the extraction deflection 4 before the particles collide against the inner wall of the vacuum duct. The orbit locally moved by the bump magnets is referred to as a bump orbit. FIG. 2 shows a bump orbit 11 linearly indicated between the bump magnets 61 and 62. In FIG. 2, a numeral 20 denotes an electrode of the extracting deflector 4, in which the resonated particles, that is, the particles having an amplitude of increased oscillations, are extracted from the electrode 20 to the outside. In FIGS. 1 and 2, two bump magnets for extraction are provided. In place, four or five bump magnets may be used. The bump orbit 11 is moved in the extracting process in order to keep the orbit of the beam extracted by the extracting deflector 4 constant. Hence, plural bump magnets operate to change the strengths of their magnetic fields in the process of extraction, respectively.
On the other hand, as the prior art, there has been proposed a method for increasing the amplitude of the betatron oscillations and thereby bringing about resonance while keeping the tune constant, that is, each strength of the magnetic fields of the quadrupole magnets 5 and 6 constant. The apparatus arrangement for this prior art is shown in FIG. 3. This apparatus arrangement is different from that shown in FIG. 1 is provision of a unit for applying a radio frequency 14. As described in U.S. patent application Ser. No. 07/958,161 Kazuo Hiramoto et al., filed Oct. 8, 1992, now U.S. Pat. No. 5,363,008, all disclosure thereof being incorporated herein by reference, the apparatus is arranged to control the tune to be constant, that is, the excitations of the quadrupole magnets 5 and 7 to be constant, or exciting the multipole magnet 9 for exciting resonance, and applying a radio frequency to a beam through the effect of the unit 14 for the purpose of increasing the amplitude of betatron oscillations and thereby causing the resonance. By this operation, this apparatus enables to extract a beam having a small diameter. When the beam is extracted, like the prior arts shown in FIGS. 1 and 2, the bump magnets are excited so as to form a bump orbit.
The foregoing prior art has the following problems.
As a first problem, the accelerator is made larger because lots of quadrupole magnets are required to be installed.
As a second problem, the control is made complicated, because lots of quadrupole magnets are required to be controlled.
As a third problem, the bump magnets are required to be provided for amending the change of an orbit of an extracted beam. This enlarges the accelerator more. Further, the associative control of the bump magnets is made complicated in the process of extracting a beam.